The present invention relates generally to the field of epoxy hardeners and in particular to new and useful groupings of epoxy hardener compositions which rapidly cure an epoxy over a range of temperatures.
There are two general types of polymers; thermoplastic and thermosetting. Thermoplastic polymers melt on heating and solidify on cooling. They can be remelted and resolidified repeatedly within limits. Thermoplastics are commonly molded by injecting a hot, molten thermoplastic into a cold steel die and removing the part from the mold as soon as the part has cooled enough to prevent distortion. The high melt viscosity of thermoplastic resins requires very high injection pressures, commonly 10,000-20,000 p.s.i. and above. These high injection pressures require enormous clamping forces to hold the two halves of the die together and so part size is limited by the clamping capacity of the molding machine. High capital investment and large production runs are characteristic of thermoplastic molding processes.
Thermosetting polymers do not melt on heating. They soften only and, if heated sufficiently will char. Thermosetting resins such as epoxies are molded by either injecting or placing by hand a prepolymer mixture consisting of epoxy resin, hardener, catalyst and various modifiers and fillers into a mold and heating for a time sufficient to complete the chemical reactions between the epoxy resin and the hardener, resulting in a thermosetting polymer having the shape and size of the mold. Molding times are considerably longer than for thermoplastic molding processes, typically five to fifteen minutes. However, injection pressures are low and so the mold clamping forces needed are also low. Thus, thermoset molding processes are characterized by low production rates and relatively low capital investment. But, large part sizes are possible.
The curing reactions of epoxies are exothermic and cure times and temperatures are determined by the heat transfer rate from the mold to the part and the scorch temperature of the epoxy. Epoxy/hardener systems which cure at low temperatures and which develop low exotherm as a result of chemical factors are very advantageous since they can be cured faster and will result in higher production rates.
Epoxies are used extensively as thermosetting adhesives for bonding wood, glass, ceramics and metals. For hand application, the epoxy resin and hardener are usually supplied in two separate syringes which have a common plunger. Pressing the plunger releases the correct proportions of epoxy and hardener. The two compounds are mixed with a spatula and applied to the bonding surfaces and then cured either at room temperature or at elevated temperature, depending on the application. Epoxy hardeners which cure rapidly at low temperature develop higher bond strength due to lower shrinkage stresses and permit higher production rates with lower energy expenditure.
Epoxy adhesives are frequently used in industrial processes in the form of xe2x80x9cfilm adhesivexe2x80x9d. A prepolymer mixture of epoxy, hardener, and other desired components is applied as a coating onto a polymer film substrate, rolled up and stored in a freezer to stop the chemical reactions between components. When needed, the film adhesive is removed from the freezer and applied to a metal or composite part, the backing is stripped off and the assembly completed and cured in an oven or autoclave.
At the point when the adhesive is removed from the freezer, the epoxy mixture begins to cure slowly at room temperature. After a certain time called the xe2x80x9cout timexe2x80x9d, the adhesive will become stiff and brittle and unusable. Latent mixtures having long out times are highly desirable in order to complete complex assemblies before curing.
Epoxies are combined with fiberglass or carbon fiber in the manufacture of composite materials. These are used extensively in military and aerospace applications, civil aircraft, sporting goods such as fishing rods, golf club shafts, tennis rackets, bows and arrows and the like. These are manufactured either by automated processes or by hand layup. Epoxies which develop excellent strength and toughness after curing at room temperature or low temperature result in composite structures having superior properties, higher production rates and lower cost. The absence of noxious vapors from the epoxy-hardener mixture is of great benefit to persons who must handle these materials.
Another application involving composites is the use of composite tooling for formed sheet metal parts. These are practical for prototyping and short production runs as a substitute for metal tools. The completed tool must be strong and hard and must cure effectively at room temperature.
Epoxies are used extensively in the xe2x80x9cpottingxe2x80x9d of S electronic circuits which are exposed to shock, vibration, and rain, for example, for protection of the circuits. The circuit is assembled and placed in a case and the liquid epoxy mixture is poured into the case, thus enclosing the circuit components and isolating them from the atmosphere as well as protecting them from vibration and shock. These are used in automobile and truck engine computers, aircraft, tanks, missiles, etc. The epoxy mixture must have a low viscosity to fill the spaces around the components before hardening. A low cure temperature is desirable to protect is the electronic circuits from heat damage and to limit shrinkage which stresses components and connections. The cured epoxy must be strong and tough to resist mechanical stresses and the cure rate should be rapid to realize a high production rate.
Electronic components are xe2x80x9cencapsulatedxe2x80x9d by dipping them into an epoxy prepolymer mixture, draining off the excess resin and curing the coating. This protects the components from atmospheric exposure. A high cure rate at low temperature is desired to prevent heat damage, keep stresses low and achieve a high production rate.
Coating systems have been developed which are used to protect metal surfaces from rust and corrosion and to enhance appearance. These are used extensively in large appliances such as washing machines, dryers, refrigerator cases, large structures such as bridge beams and architectural applications. While epoxies have enjoyed a long period of success in these applications, they have been recently partially replaced by the tougher polyurethanes. Polyurethanes have some disadvantages such as sensitivity to the resin/hardener ratio and the isocyanate resin is itself susceptible to degeneration from atmospheric moisture. Nevertheless, sophisticated metering and spraying equipment has been developed for these materials. Epoxy systems having superior strength and toughness after curing at low temperatures as well as relatively low sensitivity to the resin/hardener ratio and low toxicity may permit epoxy coating systems to regain some of their lost market share.
Prior art hardeners for epoxies are disclosed in art, such as in U.S. Pat. No. 3,812,202, which teaches a two part self-hardening epoxy composition which is formed by a phenol precursor combined with a methylol acrylic polymer. The phenol precursor is made by combining bisphenol A with a polyepoxide compound to create a composition having two or more phenolic groups. The methylol acrylic polymer can be formed by polymerizing acrylamide or diacetone acrylamide with other ethylenically unsaturated monomers, followed by adding an aldehyde, such as formaldehyde, and optionally, a catalyst. The phenol precursor and methylol acrylic polymer are mixed to a desired viscosity, applied, and heated to at least about 300xc2x0 F. to cure.
U.S. Pat. No. 4,866,133 discloses a curing agent for an epoxy containing a polymeric phenol and a polyamine. The curing agent is provided as a powdered latent curing agent mixed with a liquid epoxide resin. Polyamines used in the curing agent include diethylenetriamine and triethylenetetramine, among others. The polymeric phenols include different novolaks prepared from bisphenol A and formaldehyde, a novolak prepared from p-cresol and formaldehyde and a poly(p-vinylphenol), among others. The curing agent is activated by heating to at least about 60xc2x0 C.
U.S. Pat. No. 5,107,036 teaches a curing agent for epoxy which is a combination of two phenol compounds. One phenol is a polyhydric phenol, formed from a condensation reaction of a phenol having at least one phenolic hydroxyl group with a hydroxybenzaldehyde compound. The hydroxybenzaldehyde used in the condensation reaction must have a hydroxyl group and an aldehyde group bonded to a benzene ring, which may be substituted with at least one other constituent. The other phenol is a dihydric phenol, such as catechol, resorcinol, and bisphenol A.
Mixtures of bisphenol A and an aliphatic polyamine are disclosed in U.S. Pat. No. 4,221,890. In one embodiment, butyl glycidylether is added to the mixture which may result in the conversion of some of the bisphenol A to a secondary polyol, as well as the formation of adducts of the polyamine with the monoepoxide. There is no appreciation for the exothermic nature of the reaction between bisphenol A and the polyamine. Further, there is no consideration of the use of methylol-functional hardeners for epoxy resins, either alone or in combination with other types of polyols.
Clearly, there are many uses for epoxies and epoxy systems, and so hardeners which can more rapidly cure epoxy without charring or resulting in unstable compositions are desirable and useful.
It is an object of the present invention to provide useful hardener compositions for epoxies which cure over a broad range of temperatures.
It is a further object of the invention to provide hardener compositions for epoxies which cure fully and more rapidly at lower temperatures than existing hardeners.
It is a further object of the invention to provide hardener compositions for epoxies which have longer periods of latency at room temperature while retaining the ability to cure at lower temperatures than existing hardeners.
It is a further object of the invention to provide hardener compositions for epoxies which develop lower exotherm during curing than existing hardeners.
It is a further object of the invention to provide hardener compositions for epoxies which can be used without yielding noxious or harmful fumes.
Accordingly, three classes of epoxy hardeners which have increasing cure temperatures are provided. The Class I epoxy hardeners cure at temperatures between 20-50xc2x0 C., Class II hardeners cure between 65-100xc2x0 C. and Class III hardeners cure at 120xc2x0 C. Class I hardeners contain a mixture of polyols, polyamines and tertiary amines, while Class II hardeners have the same polyols mixed with one or more tertiary amines and Class III hardeners contain the same polyols combined with either imadazole or dicyandiamide and optionally, a tertiary amine.
Polyols which are used to form each of the different classes of epoxy hardeners are classed into two groups: group A consisting of polyols with phenolic hydroxy groups, secondary alcohols or combinations thereof, and group B consisting of polyols having methylol functional groups, secondary alcohols or combinations thereof. Epoxy hardener compositions according to the invention will contain one or more polyol from group A and one or more polyol from group B with the other elements required by the class of hardener being created.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying descriptive matter in which a preferred embodiment of the invention is set forth.
In the description of the invention, the following standard notation and definitions are used.
The notation A.B means a mixture of A and B in 1:1 molar ratio and assumed to have intermolecular bonding forces which results in a particular physical form of the product such as a medium viscosity liquid or a low melting crystalline solid. These are also known as molecular complexes.
The notation xA.yB means a mixture of A and B in molar ratio x:y.
The notation A/B means an adduct or a product produced by the chemical reaction of A and B regardless whether the reaction is addition or condensation. The nature of the reaction will be understood by a reader having ordinary knowledge of chemistry.
The notation xe2x80x9cphrxe2x80x9d means parts per hundred resin.
Cure times are expressed as cure temperature in degrees Celsius over a time period as xc2x0 C./hour.
xe2x80x9cPolyolxe2x80x9d is defined as a material having at least two reactive groups consisting of phenol, methylol or secondary alcohol and combinations of these. The polyols used in the hardeners are defined more particularly below.
xe2x80x9cPolyaminexe2x80x9d is defined as an amine chosen from one of ethylenediamine, diethylenetriamine, triethylenetetramine and tetraethylenepentamine.
xe2x80x9cTertiary aminexe2x80x9d is defined as a material having at least one tertiary nitrogen atom and may contain in addition, one or more reactive hydrogen atoms in the form of a phenol, secondary alcohol, secondary amine or primary amine. If there are no active hydrogens, the tertiary amine is an external plasticizer. If there are one or two active hydrogens, the tertiary amine is generally an internal plasticizer.
The following abbreviations are used to indicate the identified chemical composition:
Hardeners for epoxy resins are provided which have the capability of curing at lower temperatures and shorter times and with lower exotherm than existing hardeners. They are easily handled liquid materials having a range of viscosities thus allowing wide latitude in formulating. They are made from cheap and readily available industrial chemicals, have low odor, are easily handled and non-reactive when exposed to the atmosphere. Curing reactions with epoxy resins exhibit low exotherm and give cured products having low cure shrinkage, high tensile strength and high toughness.
The hardeners in accordance with the invention are broken down into three classes according to the temperature required to give complete curing: I) 20-50xc2x0 C., II) 65-100xc2x0 C. and III) 120xc2x0 C.
Class I hardeners cure an epoxy, such as Shell EPON828 epoxy, at room temperature and give pot lives ranging from xc2xc hour to 4 hours at 20xc2x0 C. depending on the application, although at least one system according to the invention requires a 50xc2x0 C. postcure for complete curing. The Class I hardeners consist of mixtures of various novel polyols mixed with polyamines and optionally, tertiary amines.
Class II hardeners consist of mixtures of these same polyols together with one or more tertiary amines. Mixtures of these materials with an epoxy provide all of the advantages cited above but give pot lives (latency) at room temperature ranging from 1-10 days yet most of these systems cure fully at 65xc2x0 C.
Class III hardeners consist of the polyols combined together with either imidazole or dicyandiamide and, in some cases a tertiary amine. When imidazole is used, rapid curing systems are obtained which are suitable for reaction injection molding at 120xc2x0 C. or, with a slight modification, thermoset injection molding. The systems based on dicyandiamide provide very long pot lives at room temperature (latency) while curing rapidly at 120xc2x0 C. and are suitable for film adhesive applications.
Epoxy hardener compositions according to the invention contain one or more polyol from group A and one or more polyol from group B in addition to the other elements required by the class of hardener being created. The two groups of polyols preferred for use with the class I, II and III hardeners are set forth in the table below:
The group A compounds are also referred to herein as first polyols having phenolic hydroxy groups, secondary alcohols and combinations thereof. The group B compounds are also called second polyols having methylol functional groups, secondary alcohols and combinations thereof.
Although some of the compounds in the two groups A and B contain more than two hydroxyl groups, only two hydroxyl groups are reactive with an epoxy under the curing conditions for the hardener compositions, due to either steric effects or chelation. The last two items in group A are special items which contain only one active phenolic hydroxyl group, and they are included here for convenience.
The compounds in each list vary widely in viscosity and molecular weight, which provides for a broad range of hardener compositions in each class. Higher molecular weight polyols are effective tougheners for epoxies and are resinous solids or semi-solids of high viscosity, while lower molecular weight polyols are usually either low melting point crystalline solids or low to medium viscosity liquids. Both types of lower molecular weight polyols are effective reactive diluents for the higher weight polyols, which provides for control of the viscosity of the mixtures of polyols, polyamines and epoxy resin.
The polyamines used in the hardeners are selected from the group consisting of ethylenediamine, diethylenetriamine, triethylenetetramine and tetraethylenepentamine. Tertiary amines of the invention include materials having at least one tertiary nitrogen atom and many contain one or more reactive hydrogen atoms in the form of a phenol, secondary alcohol, secondary amine or primary amine. Tertiary amines without an active hydrogen are considered external plasticizers, while those with one or two active hydrogens are generally internal plasticizers.
Mixtures of the previously described polyamine or tertiary amine, Group A polyol and Group B polyol are generally easily handled, stable liquids which have all of the desirable characteristics previously described provided some simple rules for determining relative proportions are observed:
a) As a starting point, one molecule of polyamine or tertiary amine should be provided for each phenolic hydroxyl group.
b) No less than one molecule of polyamine or tertiary amine should be provided for each molecule of TMP or mono-substituted TMP.
c) One molecule of polyamine or tertiary amine should be provided for each secondary hydroxyl group.
Examples of the foregoing are as follows: BPA.2DETA, (BPA.2TMP).4DETA, (TMP/BGE).2DETA, and (ROL/2TMP).2DETA (the phenolic hydroxyl groups of ROL/2TMP are non-reactive with epoxy).
Moderate increases in the proportion of polyamine or tertiary amine do not result in major changes in cure rates or properties of the cured epoxy. For Class I hardeners, decreasing the proportion of polyamine may result in incomplete curing for some systems. If the concentration of polyamine is reduced to the 50% stoichiometric level, addition of an effective tertiary amine may be required to achieve satisfactory room temperature cures.
The relative proportions of Group A and Group B polyols influence the cure rate and the mechanical properties of the cured epoxy. The high molecular weight polyols in these groups should be used at concentrations of at least 5-10 phr in order to achieve significant improvements in strength and toughness. The lower molecular weight polyols are used as reactive diluents to control mixture viscosity as well as to provide low viscosity mixtures for casting and potting applications when used as the sole polyol component. In general, the higher molecular weight polyols cure more slowly than those of low molecular weight. Cure rate is also influenced by the nature of the hydroxyl groups. Cure rate decreases in the order; phenolic hydroxyl greater than methylol greater than secondary alcohol.
Determination of the theoretical hardener concentration in the epoxy (phr) is a relatively simple procedure. First, the total molecular weight of the hardener complex is determined. Second, the total number of functional groups (n) is added up. All of the polyols with the exception of items 7 and 8 of Group A are effectively difunctional. Third, the total molecular weight is divided by the total number of functional groups which is then divided by the epoxy equivalent weight which for EPON828 epoxy is 190. The result, when multiplied by 100 gives the hardener concentration in phr. As an example, consider BPA.2TMP.4DETA:
This value should be considered a theoretical minimum concentration.
Optimum mechanical properties will be obtained when the theoretical minimum is increased by a factor which can range from 1.2 to greater than 2.0. This forces the polyamine to assume a lower functionality than the theoretical value and results in a more flexible structure with greater opportunities for hydrogen bonding. The ultimate glass transition temperature of the cured polymer will also be reduced. As a general principle, the best mechanical properties will be obtained when the glass transition temperature actually obtained by the selected cure schedule is close to the ultimate glass transition temperature (that obtained by curing at elevated temperature for an extended time). It is generally agreed that glass transition temperatures are limited to about 45xc2x0 C. above the cure temperature.
The optimum hardener concentration should be determined by testing. The mechanical properties of the cured epoxy are remarkably insensitive to excessive hardener concentrations. In tests where 2xc2xd to 3 times the theoretical hardener concentration was used, unreacted hardener was exuded from the cured epoxy, appearing as a tacky, resinous surface film which could be easily removed by wiping with a damp cloth, while the strength and toughness of the cured epoxy were still quite good.
When a polyamine or tertiary amine is combined with a polyol under appropriate conditions, an exothermic reaction occurs with the formation of a molecular complex. The mount of heat released decreases in the order phenolic hydroxyl greater than methylol greater than secondary alcohol. When the complex is subsequently combined with an epoxy resin, a rapid reaction occurs which involves the decomposition of the complex, mutual catalysis of the reactions between the epoxy and amine and also between the epoxy and the polyol and release of heat as a result of these epoxy reactions. Some of this heat is absorbed by the decomposition of the complex, resulting in a lower overall exotherm and a rapid cure without xe2x80x9crunaway exothermxe2x80x9d, which can result in foaming, vapor evolution and even charring in conventional epoxy hardener systems.
A brief discussion of the chemicals and reactions involved with the formation of the epoxy hardener compositions of the invention follows.
Trimethylolpropane (TMP) used in the epoxy hardeners is a cheap, non-toxic, low melting point crystalline solid which is incompatible with diglycidylether of bisphenol A (DGEBA). TMP is presently used as a hardener for flexible polyurethanes. Clear solutions with DGEBA may be obtained by heating the mixture above the melting point of TMP, but the mixture becomes milky on cooling to room temperature and gross phase separation occurs on standing. TMP is unreactive with epoxy absent a suitable catalyst.
However, if a tertiary amine is added to the mixture at a temperature between 50-90xc2x0 C., the mixture clears rapidly and cures to a clear, colorless solid having a low glass transition temperature of about 90xc2x0 C., depending on the concentration of TMP. Testing indicates that only two of the three methylol groups are reactive with epoxy, probably due to the formation of a strongly bonded chelate structure between the remaining methylol and either of the two adjacent ether groups. Other experiments have shown that very high glass transitions can be achieved by curing at 120xc2x0 C. in the presence of a good catalyst such as imidazole, but the cured material is very hard and brittle.
TMP is also reactive with phenyl ring hydrogen, given a suitable catalyst, either acid or base. This reaction is relatively slow compared to the reaction of TMP with epoxy groups and occurs at a higher temperature. These two reactions of TMP allow the preparation of the long chain polyols in group B which have unique properties as epoxy hardeners/tougheners leading to excellent strength and toughness of the cured epoxy, while providing rapid rates of curing at low temperatures.
Polyamines and tertiary amines form complexes with phenolic hydroxyls, methylols and secondary alcohols, the bond strength and degree of exotherm resulting from these reactions decreasing in the order phenolic hydroxyl greater than methylol greater than secondary alcohol. When the polyol contains more than one type of hydroxyl group, rapid and complete reaction between the epoxy and the polyol can be assured if the hydroxyl group which forms the strongest intermolecular bond with the amine is placed at the ends of the polyol, thus ensuring that the polyamine or tertiary amine will be located at the same positions. For example, a polyol containing two terminal methylol groups and one or more interior secondary alcohols will react first at the terminal groups and secondarily at the interior hydroxyl groups only if there are available epoxy groups remaining and the structure has not become too rigid at the selected cure temperature. If a sufficiently high concentration of polyol containing internal secondary hydroxyl groups is used, these internal groups will not react with the epoxy.
Methylols and secondary alcohols are latent with epoxy resins in the absence of a suitable catalyst. Extended latency at room temperature is a highly desirable property for many applications, such as adhesive bonding, casting and potting. Using methylols and secondary alcohols together with a latent catalyst, such as a blocked tertiary amine or an insoluble amine which dissolves at the cure temperature, useful latent formulations can be obtained.
Bisphenol A is not latent with epoxy resins but it is readily converted to a latent form by reacting it with two moles of TMP via ring substitution in the 2- and 2xe2x80x2-positions. For example, a mixture of Shell EPON828 epoxy and the adduct of bisphenol A with two moles of TMP was heated to 100xc2x0 C. for 24 hours with no detectable increase in viscosity. Similarly, resorcinol can be converted to a latent form by reacting it with two moles of TMP via ring substitution primarily in the 4- and 6-positions. This material is also a good solvent for dicyandiamide as well as other polar materials.